Replaceable cathode for electron beam generating system



Nov. 11, 1969 E. MEYER ETAL 3,478,244

REPLACEABLE CATHODE FOR ELECTRON BEAM GENERATING SYSTEM Filed Dec. 13, 1966 9 Sheets-Sheet 1 In van/00:

EDGAR MEYER WILHELM SCHEFFELS Nov. 11, 1969 E. MEYER ET AL 3,478,244

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REPLACEABLE CATHODE FOR ELECTRON BEAM GENERATING SYSTEM Filed D66. 15, 1966 9 Sheets-Sheet 5 M m 104 x l 6/ lave/210m:

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Nov. 11, 1969 E. MEYER ETAL 3,478,244

REPLACEABLE CATHODE FOR ELECTRON BEAM GENERATING SYSTEM Filed Dec. 13, 1966 9 Sheets-Sheet 4 lave/210m:

EDGAR MEYER WILHELM SCHEFFELS Nov. 11., 1969 I E. MEYER ETAL 3,478,244

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EDGAR MEYER WILHELM SCHEFFELS JM4 W United States Patent U.S. Cl. 313237 39 Claims ABSTRACT OF THE DISCLOSURE In an electron beam generating system having a replaceable cathode, the Wehnelt cylinder, the anode and the electron optical system are arranged in the housing of the system in fixed positions relative to one another and to the housing. The cathode holder and cathode may be replaced through an opening provided by means of a detachable housing portion, and the cathode holder and cathode are insertable in a mounting which is fixedly located with respect to the Wehnelt electrode and has locating means to impart to the cathode holder and cathode a predetermined operating position relative to the Wehnelt electrode.

The present invention concerns an electron beam generating system having a beam source comprising a replaceable cathode holder and cathode, a Wehnelt electrode and an anode, in which the Wehnelt electrode or a structural part rigidly connected thereto serves as a support for the cathode holder, there being also an electron optical system and an evacuable housing which has a readily detachable housing part enabling the cathode holder and cathode to be changed.

In an electron beam apparatus as used, for example in electron beam Welding, cutting and milling machines or the like, the cathode, and the Wehnelt electrode are usually connected to a high negative potential, for example -150 kv. relative to ground, whilst the remaining parts of the installation are connected to ground potential. Since the Wehnelt electrode and the cathode form the starting point of the electron beam, these parts are often combined in known installations to form a beam head. The high tension carrying parts of the beam head are assembled at the inner end of an insulating core projecting into the apparatus housing and high tension leads extend from the outside through insulating cores, usually filled with oil to the electrodes of the beam head by means of sealed in wires. The insulating core may form a disengageable socket connection with the end of a high-tension cable, which connection is preferably formed so as to be shockproof. To enable the cathode to be readily changed in such known electron beam apparatus, the insulating core and the beam head secured thereto are mounted on a readily detachable housing part, which may be hinged, so that after destroying the vacuum in the housing, the readily detachable housing part is released, permitting the cathode to be changed by means of the freely accessible parts of the beam head. Thus, it is not necessary for the flexible high-tension cable to'be uncoupled.

It has been proven in practice that in an electron beam apparatus of the kind referred to, the beam has to be readjusted after each cathode change. The reason for this readjustment is that the detachable housing part, which naturally is relatively large, after replacement on the housing and subjection to the influence of forces acting thereon during the subsequent evacuation, does not return exactly to the same position relative to the remaining parts of the installation which the housing had before the cathode change. The necessary adjustments require a certain expenditure of time, which constitutes a loss of possible operating time and impairs the economic value of the electron beam apparatus. In addition, there is the fact that owing to the relatively high weight of the beam head and of the detachable housing part, a special lifting tackle usually has to be installed, and difficulties may be encountered in carrying out the work of changing the cathode at the exposed beam head, owing to the restricted spatial conditions at the site of use of the apparatus.

The object of the invention is to provide an electron beam generating system in which the cathode may be changed in a simple manner and in which the loss of operating time involved by the cathode change is substantially minimized.

According to the present invention, in an electron beam generating system of the kind referred, the Wehnelt electrode to the anode and the electron optical system are arranged in the housing with fixed positions relative to one another and to the housing, the support including a mounting for the cathode holder, said mounting being arranged at a fixed relationship with the Wehnelt electrode, and having locating means, which impart to the cathode holder a predetermined operating position relative to the Wehnelt electrode when inserted in the mounting, and the readily detachable housing part is located at a point of the housing in the insertion path of the cathode holder and so dimensioned that the cathode holder may be inserted in the mounting through the opening formed by removal of the readily detachable housing part.

In an electron beam generating system in accordance with the invention the readily detachable housing part no longer supports the beam head and therefore is lighter in weight. It is only necessary, on changing the cathode to remove the said part, whereupon the cathode holder is slid out of its mounting in the Wehnelt electrode and removed from the housing through the housing opening. This may be effected without the aid of special lifting tackle.

Since the cathode holder is then completely separated from the machine, it may be fitted with a new cathode at a work place remote from the machine. The cathode holder may then be reinserted in its mounting through the housing opening. Due to' the locating means, an accurate predetermined operating position of the cathode holder and cathode is automatically obtained, so that readjust ment of the beam after renewed operation is not necessary. If the current supply to the cathode holder is made via plug contacts, no further operations are required; it is only necessary for the cathode holder to be returned to its mounting. The readily detachable housing part is conveniently and quickly replaceable by hand, due to its light weight and its small dimensions.

The advantages described are of particular significance if it is important to maintain the beam generating parts within close tolerances. Usually, an electron optical system is used in succession to the beam generating system to form the beam. Such electron optical systems are particularly sensitive to variations of adjustment of the beam generating system. The invention is of particular importance in connection with such apparatus.

The described electron beam generating system is such that at least one replacement cathode holder with completely assembled cathode may be kept ready as spare and, if a cathode change becomes necessary, may be changed for the cathode holder in use. With this method of operation the expenditure of time on the actual cathode change is so slight that the period that the machine is out of action is determined mainly by the pumping time required for renewed operation.

Reference should now be made to the accompanying drawings in which:

FIG. 1 is a vertical section on a reduced scale through the upper portion of an electron beam generating system in accordance with the invention;

FIG. 2 is a plan view of the Wehnelt electrode in the direction of the arrow II of FIG. 1. The cathode holder and auxiliary fittings are not shown;

FIG. 3 is a cross-section through the Wehnelt electrode on the line IIIIII of FIG. 1 without cathode holder and fittings;

FIG. 4 shows on an enlarged scale a vertical section through the lower portion of the Wehnelt electrode with cathode holder inserted, in a view corresponding to FIG. 1.

FIG. 5 shows the apparatus of FIG. 4, but rotated through 90 about a vertical axis;

FIG. 6 is a sectional view on the line VIVI of FIG. 4;

FIG. 7 is a vertical section through the upper portion of an alternative embodiment of an electron beam generating system;

FIG. 8 shows the cathode holder of FIG. 5 in a section on the line C-C of FIG. 7;

FIG. 9 shows a side view of the cathode holder used in FIG. 7, turned through 90 about the vertical axis relative to FIGS. 5 and 6;

FIG. 10 is a section on the line AA of FIG. 9;

FIG. 11 is a section on the line DD of FIG. 9;

FIG. 12 is a plan view of the base plate shown in section at the bottom of FIG. 9;

FIG. 13 is a side view, partly in section, of a gauge device for mounting a cathode on a cathode holder;

FIG. 14 is a perspective view of part of a cathode holder;

FIG. 15 is a perspective view of an alternative form of a cathode supporting member;

FIG. 16 is a side view of an embodiment of a cathode holder in accordance with the invention;

FIG. 17 is a side view of the cathode holder of FIG. 16 turned through 90;

FIG. 18 is a section on the line AB of FIG. 17;

FIG. 19 is a vertical section of an embodiment of a socket mounting for the cathode holder shown in FIGS. 17 and 18;

FIG. 20 shows the use of a packing ring.

In FIG. 1 the upper portion of an electron beam generating system in accordance With the invention is shown in vertical section. In a housing generally denoted by 1, a beam source and an electron optical system are arranged in rigid locational relation to one another. On a mounting plate 2, which has a flange 3 formed at the edge, an accurately dimensioned insulating tube 5, is located and secured by means of screws 4a; the tube is made of ceramic or other suitable material. In the center of the circular mounting 4, concentrically with the insulating tube 5, an opening 6 is formed to receive a tubular anode 7. This anode is secured in the openings by being screwed therein. In the embodiment shown the anode 7 is hollow, with an internal cross-section increasing progressively downwards. Its smallest cross-section is located at the upper end which is protected against peak discharges to the electron source arranged thereabove by means of a radiused corona discharge guard ring 8. A screen 9 is mounted on the anode tube 7 below the supporting plate 2 with clearance therefrom. In the space between the insulating tube 5 and the anode 7 the mounting plate has several pressure equalising apertures 10 formed therein. The mounting plate 2 also has apertures 11 formed therein outside the insulating tube 5. At the upper end of the 1 and the beam voltage are combined. The lead-in conduit 14 extends into the interior of the housing 1 from an insertion point 15 up to the upper connecting point 16 located in the proximity of the Wehnelt electrode. From the connecting point 16 simple, uninsulated leads 17, 18, 19 lead to the corresponding parts of the beam generating system subject to high-tension; the leads 17 and 18 go to contacts described below for cathode heating, and the lead 19 to the (metal) holder 13 of the Wehnelt electrode 12. As protection against corona discharges, a radiused corona discharge hood 20 is placed over the high tension carrying parts. The hood has an inwardly bent lower edge which engages in an annular groove 21 of the Wehnelt holder 13 and the metal upper closure 22 of the hightension insulated lead-in conduit 14. The Wehnelt holder 13 is secured to the upper portion of the insulating tube 5, as shown in FIG. 1, by means of countersunk screws 23 round its circumference. In the embodiment shown in FIG. 1, the point of insertion 15 of the high-tension insulated lead-in conduit 14 is located in an edge section of the mounting plate 2, and the lead-in conduit 14 extends diagonally from there upwards and inwards.

An upper portion of the housing in the form of a cover hood '24 is mounted on the uper surface of the flange 3 of the assembly plate 2. The cover 24 is made of insulating material such as ceramic material or synthetic resin. It is mounted partly on the surface of the flange 3 and partly on the surface of an elastic packing ring 26 inserted in the annular groove 25 of the flange 3. Laterally, the cover 24 is guided by a vertical edge 27 of the flange 3. The wall thick ness and the dimensions of the cover are so selected that any high tension discharge from the upper end of the leadin conduit 14 and at the corona discharge protective shroud '24 is directed over the outside surface of the housing. The height of the insulating tube 5 and the length of the hightension insulated lead-in conduit 14 are so selected that a high-tension flash-over from the upper end of the lead-in conduit 14 or the corona discharge protective shroud 20 is prevented. The assembly plate 2 and the anode 7, 8 are connected to earth potential. The cover 24 is enclosed by a metal jacket 28 which is also connected to earth potential and serves mainly to screen any X-rays formed in the apparatus from external objects.

The high-tension insulated lead-in conduit 14 at its point of insertion 15 is vacuum sealed into the housing, by means of elastic packing rings 29 and 30. The upper connecting end 16 of the lead 14 is also vacuum sealed; the metal 010- sure cap 22 is sealed by means of an elastic packing ring 32 inserted in the upper edge of the outer insulating tube 31 of the lead-in conduit 14 relative to the interior of the insulating sleeve 31 and the leads 17 and 18 for the cathode heating voltage are sealed vacuum tight into the insulated closure cap 22. Since such sealing techniques are known, the matter is not further dealt with. A further housing part 33, is mounted vacuum tight 0n the underside of the flange 3 of the assembly plate 2. The vacuum seal is an elastic packing ring 34 which is mounted in a groove of the flange 3. The housing part 33 contains the electron optical system which as shown in FIG. 1 comprises an electromagnetic coil 35, an annular pole piece 36 with jacket yoke 37, and a separate annular pole piece 38. These parts are secured in a predetermined position in the housing part 33 by securing means, not shown in detail. Neither is the securing of the housing part 33 on the flange 3 shown in detail. In FIG. 1 however it is indicated how with the aid of three radial adjusting screws 39 distributed round the circumference of the housing, an alignment means for the mutual radial alignment of the housing part 33 and the flange 3 or the assembly plate 2 is formed. The housing part 33 may form an integral whole with the assembly plate 2 and the flange 3. In this case, of course, alignment means 39 are not required, but there is no adjustment between the electron optical system and the beam generating system. The housing part 33 and flange 3 may be a sliding fit to provide adjustment.

The current supply to the electromagnetic coil 35 is effected via vacuum tight leads 40 which may also lead outwards at any other point than shown in FIG. 1.

The parts beneath the housing part 33 may be of any desired form and are of no interest so far as the present invention is concerned. They will therefore not be discussed further, and are not shown in the drawings. The pump lines for evacuating the housing are connected to these lower parts. In this connection it is worth mentioning that the relatively long anode tube 7, the screen 9 and the assembly plate 2 form a screening assembly which prevents evaporated material from a workpiece from being transferred from the lower part of the apparatus into the beam generating system.

The Wehnelt electrode 12 is secured to the insulating tube 5 in an accurately predetermined position by means of its holder 13. For this purpose the holder may form an integral whole with the Wehnelt electrode, or may be formed as a tight fit therein. In FIG. 1 the upper end of the Wehnelt electrode 12 is provided with an external thread 41 and a rebate 42, and is screwed into a female thread on the holder 13. The Wehnelt electrode in the embodiment shown comprises a cylindrical tube 43 having an external thread 45 on its lower end on which a member 44, which forms part of the actual Wehnelt cylinder, is screwed. The upper end 46 of the tube 43 has an outwardly enlarged wall thickness and an increased mean diameter towards the upper end.

A cathode holder 60 is mounted in the tube 43, with a cathode 61. The cathode holder and further auxiliary items have been omitted from FIGS. 2 and 3.

As shown more particularly by the enlarged plan view of FIG. 2 and the cross-section of FIG. 3, the tube 43 is provided with two diametrically opposite axial grooves 47 and 48, which each have a widened and recessed end Section 49 and 50 respectively in the upper end section 46 of the tube 43. The grooves 47 and 48 do not extend to the lower end of the tube 43, but terminate in shoulders 51 and 52 respectively (see FIGS. 3 and 5). In one of the grooves, in this case groove 48, a fiat spring 53 extends, the upper end section 54 of which is bent outwards and is secured to the upper end face of the tube 43 with the aid of a screw 55.

Between the two axial grooves 47 and 48 a further wider axial groove 56 extends, which may also be widened and recessed in the upper end section 46 of the tube 43. At the lower end this groove 56 extends to the end of the tube 43, as shown more particularly by FIG. 4.

The cathode holder 60 of FIG. 1 is inserted from above into the tube 43, until it assumes a predetermined operating position. The tube 43, which is part of the Wehnelt electrode 12, forms a socket mounting for the cathode holder *60 which is suitably formed for this purpose.

As shown in FIGS. 1 and 4-6, the cathode holder consists of two substantially parallelopiped metal supporting members 62 and 63 having two apertures 64 and 65 mutually olfset in a longitudinal direction of square cross-section, with a longitudinal slot 66 interconnecting these apertures. At their lower end the supporting members 62 and 63 extend into roof-shaped bevelled cathode supporting faces 67 and 68 each of which is limited by a shoulder 69 or 70. Two rods 71 and 72 extend through the apertures 64 and 65 of both supporting members, the rods being made of insulating material, preferably ceramic material.

In each supporting member a screw 73 or 74 extending across the slot 66 is located in a bore 75. The part of the bore 75 located on the other side of the slot 66 is threaded to permit adjustment by screwing the screw in this thread for the parts on both sides of the slot 66 of the supporting member to be drawn together. By this means the supporting members are clamped firmly to the rods 71 and 72. This clamping of the supporting members 62 and 63 enables an accurately predeterminable spatial association of the supporting members and the rods with one another to be obtained. This operation, which is the assembly of the cathode holder, need be carried out only once; during cathode change the joints between supporting members and rods are not disturbed. The supporting members 62 and 63 firmly clamped on the rods 71 and 72 are electrically insulated from one another, since the rods are made of insulating material.

The length of the rods 71 and 72 is such that they may be inserted with slight clearance in the axial grooves 47 and 48 of the tube 43. The rods and the axial grooves form orientation means adapted to one another which impart a predetermined operating position to the cathode holder 60 inserted in the tube 43 of the Wehnelt electrode 12.

In the embodiments shown in the figures of the electron beam generating system the cathode holder is inserted from above in the direction of the beam into the Wehnelt electrode. Thus, the axis of the inserting movement and the beam axis coincide. To permit the cathode holder to be removed from its socket mounting in the Wehnelt electrode and to reinsert it in the socket mounting without necessitating the removal of the upper housing portion in form of the shroud 24, and to permit the removal of the corona discharge protective shroud 20, there is an opening 77 in the corona discharge protective shroud 20, and a readily detachable housing part 78 is arranged in the housing at a location lying in the insertion path of the cathode holder which in this case comprises a ground-in stopper. The ground surface 79 is sealed with a high-tension insulating silicon grease. Below the ground surface at least one projecting head 57 is provided to collect excessive grease. Instead of the ground-in seal, as shown in FIG. 20, a packing by means of an annular member 79a made of hightension insulating elastomeric material, e.g. silicon rubber, may be provided. The annular member is inserted between the opposite surfaces of the readily detachable housing part and the housing. Since the readily detachable housing part has no fittings or parts of the beam head secured to it, and since the housing part 78 need only be large enough to permit the cathode holder 60 to pass through, the housing part 78 is light in weight and has small dimensions, so that it can be moved conveniently by hand. The guidance of the cathode holder inserted in the Wehnelt electrode solely by the rods 71 and 72 is not sufiicient to ensure an accurately reproducable operating position of the cathode holder. It is necessary for the rods 71 and 72 to have a slight clearance in the guide tracks 47 and 48, to permit the cathode holder to be readily inserted and to prevent excessive mechanical stresses due to temperature increases occurring in operation. Therefore further locating means are provided which locate the cathode holder in two directions at right angles to one another and radially to the axis -76 of the inserting movement and in the inserting direction. In both these directions the cathode holder is acted upon by two spring devices, on the one hand by the flat spring '53, already described, in one axial groove 48 and on the other hand by two contact springs 80 and 81, which are fitted on a contact plate 82, made of insulating material, and arranged in the axial groove 56 with clearance from the inner wall surface of the tube 43. The plate is secured to the lower end of the tube 43 by means of washers 83 and screws 84 and 85.

The contact springs 80 and 81, when the cathode holder is inserted, are supported against the surfaces of the supporting members 62 and 63 opposite the head of the screw 74, so that one contact spring 80 presses against one supporting member 63 and the other contact spring 81 presses against the other supporting member 62. This causes the supporting members to be urged away from the contact springs, and the guide rods 71 and 72 are urged against the side surfaces 86 or 87 of the axial guide grooves 47 or 48. These side surfaces thus act as locating surfaces in the direction of the force action of the contact springs 80 and 81, whilst in the radial direction normal thereto the central surface of the guide groove 47 opposite the plate spring 53 acts as a locating surface. Finally, the third locating surface in the beam direction consists of the shoulders 51 and 51 forming the end of the axial grooves 47 and 48. For this purpose the areas of the supporting members 62 and 63 adapted to co-operate with the contact springs 80 and 81 are so formed that the contact springs exert a force component towards the shoulders 51 and 52 on the inserted cathode holder, the surfaces of the supporting members on which the contact springs press, being bevelled. As shown in FIG. 4 the surface engaged by the contact spring 80 comprises a first surface section 88 which, in the direction of the inserting movement, extends obliquely away from the axis of the inserting movement, and a second surface section 89 adjacent therebelow which in the inserting direction, extends obliquely toward the axis of the inserting movement. FIG. 4 shows that the contact spring 80 forms a snap-in device for the cathode holder 60. With the double contact spring 80 as shown, first the upper portion and then the lower portion of the contact spring 80 is traversed by the hump between the surface sections 88 and 89, and each time a previously upwardly directed force component reverses and becomes a downwardly directed force component. In the operating position shown in FIG. 4 the contact springs 80 (and 81 not shown) exert a force both to the right and downwardly, directed against the cathode holder. Owing to this, and due to the action of the plate spring 53 shown in FIG. 5, the cathode holder is urged in three directions at right angles against locating surfaces and thus is retained accurately in its operating position.

Other embodiments of spring devices are conceivable which urge the cathode holder in a three dimensional manner against locating surfaces. The embodiment shown is simple inasmuch as the locating springs 80 and 81 simultaneously serve to supply the cathode heating current and the beam current. The current supply to the cathode holder 60 is effected therefore via spring loaded contacts, i.e. the contact springs 80 and 81 which, when the cathode holder is inserted in its socket mounting in the Wehnelt electrode, close automatically. It is obvious that other embodiments of contact springs 80 and 81 than that shown in the figures may be used. In the embodiment as shown in FIG. 4 the bent contact spring is secured by means of two screws 90, 91, nuts 92, 93 and washers 94, 95 to the contact plate 82; other methods of mounting are possible such as riveting.

The cathode 61 in the embodiments shown consists of a preformed V-shaped tungsten strip. The shanks 96, 97 of the V serve as connecting sections and the flat 98 connecting the shanks forms the actual emission surface. As further shown in FIGS. 4 and 5, the cathode 61 is mounted on the cathode bevelled supporting parts 67 and 68 of the supporting members 62 and 63 with its ends supported against the shoulders 70 and 71. The connecting sections 67 and 68 are clamped firmly in position by clamping levers 99 and 100. Each of the clamping levers is rotatably mounted in pedestal bearings 101, 102 by means of bearing pins 103, 104 and by tightening a clamping screw 105, 106, supported against the supporting member 62, 63, are pressed against the mounted cathode. Since the two scpporting members are electrically insulated relative to one another and relative to the Wehnelt electrode and are connected to the contact springs 80, 81, the cathode is connected electrically to the contact springs 80 81. The connections between the contact springs 80, 81 and the corresponding wires of the lead-in conduit 14 is effected by means of the leads 17 and 18 (FIG. 1), which pass out via an insulating support 107 and are connected by welding to the central parts of the contact springs 80, 81 (FIG. 4).

When clamping a preformed cathode 61 to the cathode holder it is important that the cathode is mounted at an accurately determined point of the roof-shaped cathode supporting members 67, 68. This may be effected with the aid of a gauge device in which a cathode is kept ready; the gauge is placed on the cathode holder and the cathode is thus mounted in an accurately defined position on the cathode supporting members. A simple form of such a gauge comprises a V-shaped cap-like template which may be fitted accurately on the cathode supporting members having a sliding-fit slot for the cathode so that after the cathode has been clamped firmly in position, the gauge may be pulled off.

FIG. 13 shows such a gauge. This gauge contains a gauge mounting 801 for a cathode holder 802. The gauge mounting 801 is provided with location means (not shown) which correspond to the location means (not shown) of the cathode holder and which imparts to the cathode holder 802 an accurately predetermined position. The gauge mounting 801 is arranged in a body 803 which 13 secured in the base frame 804. The base frame supports a cantilever arm 805 which is displaceable between two stops 807, 808. In one end position, at the stop 808, the protruding portion 809 of the cantilever arm 805 is removed from the insertion path into the gauge mounting 801, so that a cathode holder which is to be fitted with a cathode, may be inserted in the gauge mounting until it assumes the accurately predetermined position shown in FIG. 13. Then the cantilever arm 805 is slid in to the left up to the stop 807. A profiled projection 810 located on the cantilever arm and having a cross-section corresponding to the V-shape of a preshaped cathode strip to be inserted, then assumes an accurately predetermined position above the cathode supporting members 811 of the cathode holder 802 located in the gauge mounting 801. On the protruding portion 809 of the cantilever arm 805 a pressure application lever 812 is rotatably mounted in a bearing 813 and is pretensioned in position by a fiat spring 814, its cranked front end 815 pressing against the projection 810. At this point a shallow groove is formed in the projection 810 in which a cathode strip 816 is inserted with sliding fit; by depressing the rear end 819 of the pressure application lever 812 the front end 815 of this lever is lifted, the clamping levers 817 of the cathode holder having been lifted off the cathode supporting members 811 by slackening the clamping screws 818. After insertion of the cathode strip 816 in the shallow groove of the projection 810, so that its V-shanks assume a position as shown in FIGS. 4, 5, 7 and 8 over the roofshaped bevelled surfaces of the cathode supporting members, release of the rear end 819 of the pressure applicat1on lever 812 causes the front end of the pressure application lever 812 to be pressed against the fiat of the cathode strip providing electron emission, so that this flat, the position of which determines the emission centre of the cathode, is retained in an accurately predetermined position relative to the cathode holder. Then by tightening the clamping screws 818 the clamping levers 817 are pressed against the V-shanks of the cathode strip 816, so that the cathode is clamped firmly in an accurately predetermined position for its central web portion to the cathode holder.

There is a clearance, between the underside of the profile projection 810 and the cathode holder. Below the gauge mounting there is a device for lifting the cathode holder. In the embodiment as shown in FIG. 13 the lifting device has a resiliently displaceable pushbutton 820 to which a rod 821 with a conical head 822 is secured. The conical head 822, when the button 820 is depressed, engages with the radiused end of a pin 823, so that the pin subsequently lifts the cathode holder 802 located on it out of the gauge mounting. After clamping the cathode strip to the cathode holder, the rear end 819 of the pressure application lever 815 is depressed, so that the front end 815 is lifted, then the button 820 is depressed to cause the cathode holder to be lifted and the cathode strip to leave the groove in the projection 810; then the cantilever arm 805 is pushed away in the direction of the stop 808, so that the raised cathode holder may be removed from the gauge mounting.

With the gauge mounting described, the position of the emission centre of the cathode is determined by aligning means (projection 810 with groove, pressure application lever 812), which are independent of dimensional changes which may possibly occur in the cathode supporting member, e.g. in consequence of the heating during operation, so that with each inserted cathode strip the same position of the emission centre is ensured.

Of particular advantage is a gauge device which also effects the forming of the cathode strip and in which the point is adjustable at which the shaped cathode strip is placed on the cathode supporting members of the cathode holder. In a gauge device of this kind, it is further advantageous to ensure proper alignment between the gauge device and the cathode holder by utilizing those parts of the cathode holder as reference elements, which also serve as locating reference elements in actual operation, i.e. here the regions of the rods 71, 72 which co-operate with the locating side surfaces 86, 87 of the grooves 47, 48. Instead of these it is however also possible to provide guide aids on the cathode supporting members 67 and 68 which cause the cathode to assume the correct position when being mounted. Thus it is possible, for example, to provide, in the portions of the roof-shaped supporting surfaces in the proximity of the shoulders 70, 71, two juxtaposed raised portions 70a, 70b, between which the connecting portion of the pre-shaped cathode may be inserted with a sliding fit, as shown in FIGURE 14. With thick cathode strips it is also possible to provide shallow guide grooves 70c in the roof-shaped supporting surfaces as shown in FIG. 15. Whatever the method adopted, it is understood that the cathode has to be mounted at an accurately defined point relative to the cathode supporting members.

To prevent the portions of the rods 71, 72 located between the supporting members 62, 63 from being coated with cathode metal by evaporation, a screen 108 is provided as shown in FIG. 5.

Removal and insertion of the cathode holder is performed with the aid of a key, which is adapted to engage the cathode holder. In the embodiment as shown in FIG. 4 additional openings 109 are formed in the supporting members which may serve as engaging points for a key. The key may be so designed that during the insertion of the cathode holder it also acts as a guide, more particularly when the cathode holder approaches the Wehnelt electrode. The upwardly widened and recessed grooves 47, 48 (FIG. 2) result in a reliable insertion of the cathode holder. Should the possibility of introducing the cathode holder twisted by error through 180 through the inserting axis should be excluded with absolute reliability, various steps may be taken. For example, the grooves 47 and 48 may be of different widths and the corresponding ends of the guide rods 71 and 72 may vary accordingly in thickness, so that insertion is possible only with the correct orientation. Generally it will sufiice however to apply a mark in the proximity of the readily detachable housing part 78 on the housing, which clearly points to the correct position, for example, of the screw heads 74.

FIGS. 7-12, show an alternative embodiment of an electron beam generating system in accordance with the invention which is particularly suitable for welding machines having a high beam output owing to the cooling measures taken. Hereafter in the description, parts which occur analogous to the embodiment as shown in FIGS. 1-6 are each designated with reference numerals increased by 500; new parts have reference numerals above 700. In contrast to the embodiment as shown in FIG. 1, in the welding apparatus as shown in FIG. 7 the insulating tube 505 supporting the Wehnelt electrode 512 also forms the housing wall. It is thicker than the insulating tube of FIG. 1 so as to present adequate insulation without having additional vacuum spacing. The construction has the advantage that the heat developed by the electron source during operation is readily conducted away. Moreover the dimensions of the housing are reduced.

As evident from FIG. 7 the lower edge of the insulating tube 505 has a metal annular flange 503 secured vacuumtight thereto. The insulating tube is preferably made of a moulding resin mixed with a mineral filler substance and the flange 503 is connected with the insulating tube during moulding. For this purpose the flange 503 is provided with an anchoring profile 701 which is spherical in radial section, ensuring a secure mounting in the insulating tube 505.

The inner edge 702 of the flange 503- is a sliding fit, and an outer conical surface 703 of the flange 502 is also a sliding fit. A sealing surface 704 co-operating with surface 703 is formed on the flange 503, which sealing surface has an elastic annular sealing packing 705 in a lower housing part 533. The lower housing part 533 has an inner face 706 which forms a sliding fit with the already mentioned outer face 703 of the annular flange 503. Due to this sliding fit the insulating tube 505 may be removed from the lower housing part and replaced thereon without appreciably altering the geometry of the beam producing parts. The lower housing part 533 as in the embodiment as shown in FIG. 1 assumes a predetermined locational relationship with the electron optical system and the other parts devices of the welding machine. These are not referred to further herein.

The inner face 702 of the flange 502 serves to locate a metal assembly plate 702 having pump openings 510 formed therein, the plate being centrally secured in a similar manner to the tubular anode 507 in FIG. 1, with an anode screen 508 screwed thereon. The sliding fit between the assembly plate 502 and the flange 503 is protected by an upright radiused corona discharge protective collar 707. The sliding fit at 702 may also be formed as a screw connection.

In the upper portion of the insulating tube near the inner wall surface thereof a supporting ring 708 is anchored as by moulding, the exposed inner edge of which is provided with a thread and a sliding fit 709, and serves to secure an annular Wehnelt holder 513. The Wehnelt holder 513 is formed in a conventional manner with radiused edges and contains an annular plate 710 which is pierced by openings 711. The Wehnelt electrode 512 is integral with the annular plate 710; alternatively it may be made as a separate component and fitted into the annular plate. Since when changing a cathode the mutual positions of the insulating tube 505, the anode 507, the Wehnelt electrode 512 and the lower housing part 533 do not alter, the holding means and location of these parts 1 will not be further described.

In the embodiment as shown in FIG. 7 a cathode holder 560 is used the positioning of which is effected by two knife edge bearings arranged perpendicularly relative to one another which are located in the direct proximity of the cathode. The Wehnelt electrode is formed as a socket mounting for the cathode holder. As in the embodiment shown in FIG. 1, the Wehnelt electrode comprises a tube 543 and a shaped member 544 which forms the actual Wehnelt cylinder, screwed to the tube. In the embodiment shown the tube 543 has an intermediate tube 712 inserted therein with sliding fit, which serves as support for contact mounting plates 582, 582a, of insulating material. The intermediate tube 712 has a radial shoulder 713 as stop for the upper edge of the shaped member 544, and a sliding fit 545 for the upper lateral edge of the shaped member 544. A threaded ring 714 is screwed into a recess formed on the lower edge of the intermediate tube 712. The outwardly protruding upper edge of the shaped member 544 is supported between the shoulder 713 and the threaded ring 714 at the sliding fit 545. In the upper part of the tube 543 a female thread 715 is formed to receive a threaded ring 716 which in this case presses the intermediate tube 712 with the threaded ring 714 screwed thereon and the upper edge of the shaped member 544 together to form a rigid unit. On the inside surface of the base, closing the shaped member 544 downwardly, there is also secured a washer 717 having a central hole, held in position by means of screws. As shown by the plan view of FIG. 12 and the sectional view in the lower part of FIG. 9, two grooves 718 and 719 for the knife edge bearings 730 and 731 are formed at right angles to one another in the upper surface of this washer 717.

In the embodiment shown it is not only possible for the cathode holder to be removed, but it is also possible for the parts determining the acceleration field, which are detachably arranged in their seatings, to be removed through the opening 577 in the housing closed by the detachable housing part 578. This is necessary at intervals for the purpose of cleaning. After slackening the threaded ring 716 and releasing the socket connections 734 the intermediate ring 712 with the parts secured thereto, e.g. the parts 544 and 714, may be drawn out upwardly. For this purpose a key (not shown) is used which is adapted to engage in grooves or projections (not shown) in the intermediate ring 712. After removing the intermediate ring 712 and the parts connected thereto it is also possible for the anode screen 508 to be slackened by means of a further key and pulled up through tube 543. The external diameter of the anode screen 508 is slightly smaller than the smallest free diameter of the tube 543, and it is also provided with key engaging means, i.e. the grooves 739 indicated in FIG. 7. Even after removal and replacement of the said parts of the acceleration system, the beam geometry of these parts remains unchanged.

The cathode holder comprises two supporting members 562, S63 electrically insulated from one another, which like the supporting members 62, 63 of the embodiment shown in FIG. 1 are provided with a first bevel sur face 588 and a second bevel surface 589. At the lower end the supporting members 563 extend into cathode supporting parts 567, 568 which are provided with roof-shaped bevelled supporting surfaces and shoulders 570, 571 for the support of the cathode 561. Instead of the clamping levers of FIG. 1 simple clamp plates 599, 600 are used, with clamping screws 605, 606.

The structure of the cathode holder and more particularly the retention of the two supporting members 562, 563 is evident from FIGS. 8-12. Between the supporting members 562, 563 two ceramic material members 571, 572 are inserted, of which the central sections are tapered into circular cylindrical webs 720, 721. The webs are so long that on each web two metal tightening straps, e.g. the straps 722, 723 are accommodated with mutual clearance. One end of each tightening strap is connected with a supporting member 563 and may be tightened by means of a screw 726-729. This causes the supporting members 562, 563 with the ceramic members 571, 572 to be formed into a unit. The lower ends of the ceramic members 571, 572 are formed as knife edges at right angles to one another, and fitting the knife edge bearing grooves 718, 719. The knife edge bearings and the knife edge bearing grooves form locating means whereby the location of the cathode holder relative to the Wehnelt electrode is accurately reproducable.

As evident from FIG. 7 the contact springs 580, 581 secured to the contact supporting plates 582, 582a are so arranged that, when the cathode holder is inserted, they engage with the bevel surfaces 588, 589 of the supporting members 562, 563 and in the operating position shown in FIG. 7 they exert a downward force component on the cathode holder in the direction towards the socket mounting. Lead-in wires 517, 518 for the cathode heating current are connected to the contact springs 580, 581.

Since the cathode holder is connected only by its ceramic knife edge bearings and the contact springs 580, 581 with the Wehnelt electrode and of these connections the contact springs have by far the higher degree of heat conducting ability, substantially the whole heat flow from the cathode holder, is discharged by the supporting members 562, 563 via the contact springs 580, 581 and the lead-in wires have to have a large crosssectional area, promoting heat conduction. To prevent damage to the insulating tube 505 and the other plastics material injection moulding parts it is necessary to ensure that the temperature of the components coming into contact with these members is kept sufficiently low. For this purpose the holders for the lead-in wires and the Wehnelt electrode mounted on the housing or the insulating tube respectively are formed so as to be supported over a large area so that the heat flow is also widely spread.

Accordingly the holding means for the Wehnelt electrode 512, i.e. the supporting ring 708 is shaped so as to have a relatively large contact surface with the insulating tube 505, so that local overheating of the insulating tube is prevented. The holding means, more especially the annular Wehnelt holder, act as cooling vanes owing to their large surface. Similar measures for heat dissipation are provided for the lead-in wires 517 and 518; thus the lead-in wires are each connected to a sector of a ring of cooling vanes 732, 733; the connections may be welded or established by means of socket couplings 734. The lead 519 serving to supply the Wehnelt voltage as shown in FIG. 7 is also connected to a sector of the ring of cooling vanes. In this embodiment three such cooling lugs/vanes in all are provided. These cooling vanes as evident from FIG. 7, are mounted integrally on the inside surface of the insulating tube 505 in contact with the inwardly protruding edge section 524, and may be moulded or cemented thereto. The heat fiow along the leads 517, 518 and 519 is thus distributed over the whole area of the cooling vanes, so that this temperature remains low enough to avoid any harmful local overheating of the insulating material. At all points the thickness of the insulating layer between the parts connected to high-tension and the outside earthed surface of the housing is so dimensioned that a disruptive puncture is impossible. In the embodiment shown in FIG. 7 the distance between the holders of the hot components and the outside surface of the housing does not include a vacuum gap; rather this distance is completely filled with a layer of an insulating material which is sufiiciently thick for the electric insulation and constitutes itself part of the housing.

The metal jacket 735 of the housing provided is supported close against the outer surface of the insulating body and therefore has a good heat conducting contact therewith. This jacket 735 is connected to earth potential. A radiation shield 528 may also be provided. To produce a good heat conducting contact between the radiation shield 528 and the metal enclosure 735 the radiation shield is spaced in a circumferential region located below the beam generating system from the inner housing, by means of an elastic packing ring 736, and the space 737 between housing 505 and radiation protection enclosure jacket 528 is filled with a good heat conducting medium. Alternatively a cooling fluid such as air or water may be pumped through the space 737.

Similar steps are taken to insert and remove the cathode holder, in the embodiment as shown in FIG. 7, as in the embodiment of FIG. 1. The central opening 577 defined by the edge section 524 of the insulating tube 505, and the opening left open in its centre by the sectorshaped cooling vanes 732, 733 are of such a size as to permit the cathode holder 560 to be passed therethrough without difliculty. The central opening 577 is closed by means of a readily detachable housing part 578 in the form of a ground-in stopper, the ground surfaces 579, being sealed with a highly insulating silicone grease. Suitable anchoring means are provided on the cathode holder in the form of the opening 609 shown in FIGS. 7, 9, with which a displaceable pin of a key may engage. The upper 13 surface of the housing is covered by a metal screening cover 738 of suitable thickness so that total screening of the beam generating system is obtained.

The connection to the electric supply leads connected to high potential is not shown in FIG. 7. It is obvious that connections extend from the three cooling vanes 732, 733 to a multi-core high tension connection which passes out of the side of the housing and may be formed as a socket connection.

FIGS. 16-18 show an alternative embodiment of a cathode holder which may be inserted in a socket mounting as shown in FIG. 19. This cathode holder, like the embodiments shown in FIGURES 1 and 7, also comprises two metal supporting members 901 and 902 which are electrically insulated from one another. The cathode holder has a generally cylindrical shape. One supporting member 901 has a cut-out in which the other supporting member 902 is inserted. For mutual alignment two ceramic pins" 903 and 904 are used, which are mounted with sliding fit in continuous bores of the supporting members. A ceramic plate 905 is used, for the insulation between the two supporting members. The members are held together by screws 906 and 907, which are insulated from the supporting member 902 by means of insulating washers 908 and insulating sleeves 909 (FIG. 18). The ceramic plate 905 has holes for the pins 903, 904 and the screws 906, 907. The lower ends of the supporting members are provided with roof-shaped bevelled supporting surfaces 910, 911 which end in shoulders 912, 913 and serve to support a V-shaped cathode strip (not shown). To clamp the cathode strip in position as in the embodiment shown in FIG. 5, clamping levers 914, 915 with clamping screws 916 are provided. The clamping levers are rotatably mounted at 917, 918. The lower ends of the clamping levers are recessed and the clamping jaws 919, 920 are rotatably mounted at 921, 922. At the upper end one supporting member 901 is provided with a cylindrical extension 923, in which a female thread (not shown) is formed to enable the cathode holder to be engaged by a threaded key. On the cylindrical outer surface 924 of the one supporting member 901 projections 925-928 are provided which are vertically and circumferentially offset relative to one another. In the outer surface 929 of the other supporting member 902 a horizontally extending V-slot 930 is formed at a point located substantially symmetrically to the projections 925-928 of the supporting member 901. A protruding guide pro- 'jection 931 is also formed on the supporting member 901.

The cathode holder shown in FIGS. 16-18 is associated with the Wehnelt arrangement shown in FIG. 19. This arrangement comprises an internal member 932 having a cylindrical continuous bore 933 enlarged at the upper end and a tubular outer body 934 rigidly connected with the internal member and electrically insulated from it. The Wehnelt arrangement shown in FIG. 19 is secured as shown in FIGS. 1 and 7, to the structure of the apparatus, and both the outer member 934 and the inner member 932 may be formed as fixing means. In the arrangement shown in FIG. 19 the inner member 932 is provided at its upper end with an annular flange 935 serving as a mounting. The relative positions of inner and outer members is determined by ceramic pins 936 distributed over the circumference. In the annular space 937 between inner and outer members, axially parallel ceramic spacer rods may also be provided. As shown in FIG. 19, the whole assembly is held together by a ring 939 screwed to the lower end of the inner member at 938. On the outer circumference of the outer member 934 a thread 940 is provided, serving to screw on a Wehnelt assembly (not shown), which is shaped as the Wehnelt member shown in FIGS. 1 and 7, or at 544.

I In the inner wall surface of the inner body 932 an axial groove 941 extending substantially to the centre of the inner member is formed, which when the cathode holder is inserted, receives the guide projection 931 and thereby ensures the correct angular positioning of the inner member and cathode holder. The correct position of the cathode holder in an axial direction is determined by the projections 942 formed in the inner wall surface of the inner member. These projections engage with the shoulders 943, 944 (FIG. 17) of the supporting member 901 of the cathode holder. Centering of the cathode holder in the inner member is obtained by means of a resiliently mounted ball 945 in the wall of the inner member which presses the inserted cathode holder in the direction of the opposite wall, so that the projections 925-928 (FIGS. 16-18) are in engagement with this wall region. The ball 945 is insulated from the inner member. As shown in FIG. 19, the ball945 is mounted in a seating 9% on a supporting plate 947, which is secured in a channel 948 formed in the outer wall of the inner member 932; the plate 947 is insulated by insulating material 949, 950 and fixed by means of screws 951. The accurate positioning of the supporting plate 947 is ensured by means of a ceramic locating pin 963. The seating 946 with the ball 945 engages with clearance in an aperture 952 of the inner member. On the supporting plate 947 a spring assembly 954, with two flat springs is secured by means of screws 953; this spring assembly urges the ball 945 inwardly in the bore of the inner member. The ball 945 and the V-slot 930 together form a stop device which retains the cathode holder in the centered end position when inserted in the inner member.

The inserted cathode holder and supporting member 901 are electrically connected by way of the projections 925-928 with the inner member 932, whilstthe other supporting member 902 is only in connection with the ball 945.

To ensure that the cathode heating current (and the beam current) is reliably supplied, pressure contacts in the form of resilient contact springs (not shown) are provided, which are so arranged in the Wall of the inner member 932 that, when the cathode holder is inserted, they come into contact with the supporting members 901 and 902.

In the Wehnelt arrangement shown in FIG. 19 the Wehnelt voltage is supplied to the outer member 934 via a contact pin 955. The contact pin 955 is passed through an insulating member 965. Contact pins may also be provided to supply the cathode heating current (and the beam current); in FIG. 19 such a contact pin 957 is shown, which is inserted in an annular plate 962 secured and electrically insulated by means of screws 958 and insulating material 959, 960 and 961 to the inner member flange 935. The annular plate 962 is connected by means of a lead (not shown) with a contact spring (not shown) which is supported against the supporting member 902 of the cathode holder. A further contact pin (not shown) may be provided, which is in direct connection with the inner member and the other contact lugs.

As evident from FIGS. 16-19, the Wehnelt arrangement shown therein, which is not moved during cathode change and which remains in a fixed position relative to the electron optical system of the beam, also forms a socket mounting for an interchangeable cathode holder. A cathode holder inserted in the socket mounting, i.e. the inner tube 935 rigidly connected to the Wehnelt electrode screwed on to the outer tube 934 has an accurately predetermined operating position relative to the Webnelt electrode, established by the projections 925-928, the projection 942, the guide projection 931 and the guide track 941. These elements act as locating means. The spring device 954 operating with the ball 945 causes the cathode holder to be radially urged in position and thus the position of the cathode holder relative to the beamaxis 976 is determined.

Numerous other embodiments are possible without departing from the scope of the invention. Thus instead of the knife edge mounting shown in FIGS. 7-12, a

15 three-point mounting may be provided; it is also possible for the parts not displaced during a cathode change to be formed so as to be detachable, and mounted in close tolerance holders to permit the apparatus to be dismantled lithout interfering with the structural geometry.

We claim:

1. An electron beam generating apparatus comprising a cathode electrode, a cathode holder to support the cathode, a Wehnelt electrode and an anode electrode, with said electrodes arranged to generate an electron beam along an axis,

a support mounting on the Wehnelt electrode to slidingly and removably support the cathode holder with predetermined locating of the cathode relative to the operative region of the Wehnelt electrode,

an evacuatable housing enclosing said electrodes and having a readily detachable housing part for axial removal and axial insertion of the cathode and cathode holder through an opening in the housing formed by removal of the detachable part without physical disturbance of high voltage conductors applied to the Wehnelt electrode.

2. The apparatus according to claim 1 wherein the readily detachable housing part is provided with an electrically insulating material facing the cathode, and wherein said housing is lined with an insulating material on the internal surface thereof, with peripheral facing surfaces of the insulating material of the detachable part and the housing internal surface shaped to provide an insulating distance for high-tension insulation from the cathode.

3. The apparatus according to claim 2, in which a high tension insulating paste-like sealing agent is used for vacuum sealing the readily detachable housing part to the housing.

4. The apparatus according to claim 1 wherein the cathode holder is insertable in the Supporting mounting in the direction of the beam.

5. The apparatus according to claim 1 wherein the Wehnelt electrode is provided with a guide track, and the cathode holder is provided with a guide member sized to fit within the guide track with close slidable clearance, a spring attached to the Wehnelt electrode to urge the cathode holder into a desired radial position relative to the beam axis.

6. The apparatus according to claim 5 wherein several springs are provided angularly offset relative to one another about the beam axis of the inserting movement of the cathode holder.

7. The apparatus according to claim 6, wherein said Wehnelt electrode is provided with two substantially parallel and diametrically opposite axial guide tracks along the axis of the inserting movement of the cathode holder, with one of the springs arranged in one of the guide tracks to locate the inserted cathode holder against the other guide track.

8. The apparatus according to claim 5 wherein a region of the cathode holder cooperating with the spring is so formed to establish a force component in the beam axis direction against the inserted cathode holder.

9. The apparatus according to claim 8, wherein said one cathode holder region is a bevelled surface spaced from and aligned obliquely to extend away from the axis of the inserting movement.

10. The apparatus according to claim 8 wherein said one cathode holder region comprises a first surface section arranged obliquely relative to the beam axis and spaced from the axis for normal operating contact with the spring and wherein said cathode holder is provided with a second cathode supporting surface section disposed obliquely towards the beam axis.

11. The apparatus according to claim 1 wherein said cathode holder fits with snap-in action to the Wehnelt electrode.

12. The apparatus according to claim 10 wherein the Wehnelt electrode and the cathode holder cooperate to form a snap-in connector formed of a radially compressible spring insulatively attached to the Wehnelt electrode and a downwardly slanted surface on the cathode holder in contact with the spring.

13. The apparatus according to claim 1 and further including spring means for conducting heating current to the cathode holder upon the closing of spring contacts when the cathode holder is inserted in the support mounting in the Wehnelt electrode.

14. The apparatus as recited in claim 13 wherein said cathode holder further includes a pair of insulatively spaced electrically conducting cathode supporting members located with the beam axis between them, a pair of radially compressible springs insulatively mounted within the Wehnelt electrode in compressive relationshipwith the cathode supporting members and means for conducting high voltage electrical power to said cathode members through said springs.

15. The apparatus according to claim 14 wherein the cathode is selectively preformed and is provided with connecting sections clamped firmly on the cathode supporting members of the cathode holder.

16. The apparatus according to claim 15 wherein the cathode supporting members, have a shape adapted to the selected form of the cathode connecting sections and guide means provided on the cathode supporting members for selectively positioning the cathode when secured to the cathode supporting members.

17. The apparatus according to claim 16, in which the guide means include grooves for receiving at least a part of the connecting sections of the cathode.

18. The apparatus according to claim 17 in Which the cathode locating means provides a three-point mounting of the cathode.

19. The apparatus according to claim 17, in which the cathode locating means includes two knife edge bearings extending in a plane normal to the beam and spaced from one another at angular positions around the beam axis.

'20. The apparatus according to claim 19, in which the cathode holder includes anchoring means for inserting and extracting of the holder by a key.

21. The apparatus according to claim 20, in which the locating means is positioned in the vicinity of the cathode.

'22. The apparatus according to claim 1 in a high-tension insulated lead-in conduit extending into the interior of the evacuable housing from a lead-in point to a connecting point located in the proximity of the Wehnelt electrode, the connecting point having adequate clearance relative to the parts of the housing connected to earth potential, and high-tension carrying leads, from the connecting point being connected from the conduit connecting point to the cathode holder and Wehnelt electrode.

23. The apparatus according to claim 22 and further including an insulating tube, with the Wehnelt electrode positioned therewithin, an assembly plate connected to the housing and ground potential for supporting the conduit, the insulator tube and the anode, said assembly plate further being provided with an aperture opposite said conduit.

24. The apparatus as recited in claim 23 wherein the Wehnelt electrode, the anode, the cathode holder and the cathode have a predetermined reproducible position relative to the supporting plate for accurate locating of the cathode with respect thereto.

25. The apparatus according to claim 23, wherein the housing is further provided with a vacuum-tight shroud mounted on one side of the assembly plate.

26. The apparatus according to claim 25, wherein the housing has a vacuum-tight removably mOunted housing part located in general alignment with the electron beam.

27. The apparatus according to claim 26, in which alignment means are provided between the assembly plate and the housing part mounted on the other side of the assembly plate for generally aligning the housing part with the electron beam.

28. The apparatus according to claim 27, in which the alignment means has three angularly spaced adjusting screws located substantially radially relative to the electron beam.

29. The apparatus according to claim 27, in which said aligning means includes a sliding fit between two surfaces.

30. The apparatus according to claim 23 and further including holding means for retaining the Wehnelt electrode to the insulator tube, said holding means having a contact surface with the insulating tube sized to prevent local overheating of the insulating tube due to the flow of heat from the Wehnelt electrode.

31. The apparatus according to claim 30, in which the holding means includes cooling vanes.

32. The apparatus according to claim 31, in which said cooling vanes are connected to the high tension lead-in wires for the cathode holder.

33. The apparatus according to claim 31 and further including a layer of high tension insulation connected to an outer ground potential region of the housing, wherein said vanes are connected to said layer of high tension insulation over a surface area selected to prevent overheating from heat conducted by the vanes.

34. The apparatus according to claim 33, in which the housing comprises an inner insulating member and an outer metal enclosure, with said outer enclosure being in good heat-conducting relationship with the inner insulating layer of high tension insulating material.

35. An electron beam generating apparatus comprismg an evacuatable housing,

means attached to the housing for generating a beam of electrons and directing said beam along an axis, said means including a replaceable slidingly mounted cathode,

means attached to the housing for conducting high voltage electrical power to the beam generating means from a location radially removed from the electron beam axis,

said housing being selectively removable to provide access to the replaceable cathode for axial installation and axial removal thereof without physical disturbance of said high voltage conducting means.

36. The apparatus as recited in claim 35 wherein said housing is provided with a replaceable cover over a housing opening sized for passage of the replaceable cathode, said cover and opening being positioned on the housing at a location selected to permit cathode removal without physical disturbance of said high voltage conducting means.

37. The device as recited in claim 35 wherein the replaceable cover is located on the housing in general alignment with the beam axis.

38. The device as recited in claim 37 wherein the beam generating means further includes an anode, a cylindrical Wehnelt electrode with the replaceable cathode mounted within the Wehnelt electrode and means for insulatively mounting said cathode with predetermined axial position and angular rotation relative to the beam axis in the Wehnelt electrode.

39. The device as recited in claim 38 wherein the conducting means frictionaly engages the replaceable cathode with a force having a component along the beam axis to axially firmly seat the replaceable cathode on the Wehnelt electrode.

References Cited UNITED STATES PATENTS 3,187,216 6/1965 Sciaky 313237 3,109,953 11/1963 Burnett 313236 2,305,458 12/1942 Ruska 313-237 X 2,319,061 6/1943 Hillier 313237 X JOHN W. HUCKERT, Primary Examiner B. ESTRIN, Assistant Examiner US. Cl. X.R. 

